I'm now using the CST genetic algorithm optimizer to optimize the gain of my antenna, total 16 parameters is involve in optimization.

I'm having problem on putting value on the slots for the setting window like the population size, max number of iterations, mutation rate and other setting, so what is the value that i should key in? Or just keep the default setting?

There is no correct way to use a genetic optimization algorithm. The optimal parameters are unknown, and actually, the "right" parameters depend entirely on the specific problem.

So the right way to do this, is start with the default values, and adjust them as you see fit. The tradeoffs typically are convergence time versus how thoroughly you search the parameter space.

If you want to learn what each parameter does, look it up online or check the help files to understand what they do. Genetic optimization is not a science - and the results are not guaranteed in any way to be globally optimal. But it is a tool that can be used to improve results - so just try it and see what happens.